The typical modular instrumentation system includes a mainframe (also referred to as a chassis) that interfaces with a computer. The mainframe defines numerous card slots that are connected by an instrumentation bus to the computer. Each card slot is configured to receive an instrument module, which is a hardware device configured to collect sensor data, for example. The instrument modules are typically 6U in height and C-size (unlike VME bus modules which are more commonly B-size). The instrumentation bus is configured to facilitate the transfer of the sensor data between the instrument modules and the computer. Based on the type of instrument modules installed in the mainframe, the instrumentation system is configurable to meet a particular requirement or perform a particular function. Thus, the modular instrumentation system is an alternative to interfacing numerous standalone instruments with the computer.
The current state-of-the-art for instrumentation systems includes instrumentation busses utilizing the Versa Module Europa eXtenstions Instrumentation (VXI) and the General Purpose Interface Bus (GPIB and IEEE 488) architectures. The VXI bus architecture is an open standard platform for automated testing based upon an earlier standard referred to as VMEbus. VXI provides additional bus lines, as compared to VMEbus, for timing and triggering as well as mechanical requirements and standard protocols for configuration, message-based communication, multi-chassis extension, and other features. In 2004, an extension referred to as 2eVME was added to the VXI bus specification, giving it a maximum data rate of 160 MB/s.
IEEE-488 is a short-range digital communications bus specification. IEEE-488 is an interface bus between modules as opposed to an interface bus within a larger mainframe structure. It was produced in the late 1960s for use with automated test equipment, and is still in use for that purpose. IEEE-488 was structured as a HP-IB (Hewlett-Packard Interface Bus). The IEEE-488 bus provides data as 8-bit, parallel data. The bus employs sixteen signal lines, with eight signal lines used for bi-directional data transfer, three signal lines for handshake, and five signal lines for bus management. An additional eight signal lines are included for ground return lines. The devices on the bus have a unique 5-bit primary address, limited to the range of zero to thirty, enabling up to thirty-one total possible addresses. The standard allows up to fifteen devices to share a single physical bus of up to twenty meters total cable length. The physical topology can be linear or star (forked). Active extenders allow longer buses, with up to thirty-one devices theoretically possible on a logical bus. Control and data transfer functions are logically separated; a controller can address one device as a “talker” and one or more devices as “listeners” without having to participate in the data transfer. Multiple controllers may share the same bus; but only one can be the “Controller In Charge” at a time. In the original IEEE-488 protocol, transfers used an interlocked, three-wire ready-valid-accepted handshake. The maximum data rate is about one megabyte per second. The later HS-488 extension relaxes the handshake requirements, enabling communication at rates up to 8 Mbyte/s. The slowest participating device determines the speed of the bus.
Both IEEE-488 and VXI have hardware overheads that make any instrumentation systems based on them expensive, since each instrument module must carry the overhead of a complex bus architecture. Thus, further developments for the instrumentation bus of a modular instrumentation system are desirable.